4 d seismic monitoring using horizontal wells 3

4D seismic monitoring using horizontal wells – A synthetic case study from Troll –

Eva Janská (formally: TU Delft, recently: Statoil)

S. A. Petersen (Statoil), G. Toxopeus (Statoil)

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4D seismic monitoring using horizontal wells – A synthetic case study from Troll

Eva Janska, S. A. Petersen, G. Toxopeus

We have been looking at the possibility of monitoring the gas-oil contact movements by seismic acquired in the horizontal well, in case that the sources and receivers are mounted

to the well completion.

Oil

Water

Gas

Copy: Amerex Corporation

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Frequent4D Monitoring

of Gas-Oil Contact (GOC)

• Where?

• How?

• Results

First Feasibility Study on

Presentation Outline

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Troll Field – West Oil

• Thin oil layer with a gas cap

• Oil produced by multi-branched horizontal wells

• Geology

− Semi-horizontal layers

− Calcite cemented zones of uncertain extend and continuity

− 2 types of reservoir sands

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2D Velocity Models Simulating Oil Production in Troll

Velocity difference due to oil production

Original GOC

GOC Drawdown

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How to Monitor the GOC Movements Frequently?

• Seismic acquisition

• Tools fixed in/close to a horizontal well

2nd well branch –same depth as 1st

• Receivers

− In the horizontal part of the well

− Close to the target area of GOC

• Source(s)

− Various positions & frequencies

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Frequent GOC Monitoring - Investigation Strategy

-> Reservoir Before & After Production

− Velocity models

-> Simulating Seismic Acquisition

− Source(s) & receivers

− Finite-difference approximation of acoustic wave field

− > Synthetic seismograms

-> Evaluation of Results

− Differential seismograms

• More signal ≈ More GOC drawdown

− Interpretation suggestions

− > Optimal acquisition setup

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Seismograms

Sea bed

Heel of horizontal well – Large velocity model

Heel of horizontal well

Multiple – Gas zone

Single – Gas zone

Multiple – Horizontal well

Single – Horizontal well

10 0005 0001 00050010050Frequency [Hz]

Position

Original GOC

Sea bed




Single – Gas zone



10 0005 0001 00050010050Frequency [Hz]

Position

GOC after oil production

Sea bed




Single – Gas zone



10 0005 0001 00050010050Frequency [Hz]

Position

Differential seismograms

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Results

• Best setup: − Multiple source in horizontal wells

− 1000 Hz Ricker wavelet

− Easier to interpret Harder to implement

• Most economic setup:

− Source in heel of well

− 100 Hz Ricker wavelet

− Easier to implement Harder to interpret

… The most advisable source delivers the most precise and demonstrable information about the GOC shape after oil production and its distance from the well…

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Multiple Source in Horizontal Well – 1 000 Hz~ plane wave

• GOC indication on differential seismogram:

− Direct

− Delay in reflections

• Also with the OWC movements

time

well

~distance to well

Seismogram – GOC after oil production

Differential Seismogram – effect of oil production

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100 Hz Source at Heel of Horizontal Well

• Complex origin of the time delay in signal:

− Snapshots from the wave field with original GOC (A) and from the ‘differential’ field (B)

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100 Hz Source at Heel of Horizontal Well

Max differential signal far behind max GOC drawdown.

Differential Seismogram – effect of oil production

Max GOC drawdown

Max differential signal

Differential signal assigned to the centre of source-receiver:

• Shape coincidance with GOC Calibration required

time

well

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Conclusions

4D seismic monitoring using horizontal wells- A synthetics case study from Troll -

Numerical simulations show that monitoring the gas-oil contact by a seismic acquisition, where the tools are fixed in horizontal wells,

is feasible.

Most favourable source positions for the GOC monitoring:• Proved by simulations – Multiple sources in the horizontal well• First to be implemented – At heel of the well & sea surface

Geological setting:• Not a crucial aspect (in horizontally layered reservoir)• Reflective layers

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Acknowledgments

• Statoil for the internship opportunity.

• Jan Thorbecke (TU Delft) for executing elastic simulations of some of my models.

• Guy Drijkoningen (TU Delft), Norunn Skjei (Statoil) and Kristian Gjerding (Statoil) for their help and concern during the project.

Thank you for attention !

Questions & Answers

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Eva Janska, S. A. Petersen, G. ToxopeusClassification: Internal 2010-06-11Classification: Internal 2010-05-03Classification: Internal 2010-05-03

4 d seismic monitoring using horizontal wells 3

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